Passive thrust enhancement using circulation control

ABSTRACT

A passive thrust enhancement system having a propeller, which includes a propeller hub, a first propeller blade, a second propeller blade, wherein each of the first and second propeller blades include a fluid flow channel within the first and second propeller blades, which is fluidly connected to a flow exit slot, and a flow capture device, which includes a plenum chamber, mounted on the propeller hub, wherein the plenum chamber is fluidly connected to the fluid flow channels.

CROSS REFERENCE

This application claims the benefit of provisional application Ser. No.61/560,530, filed on Nov. 16, 2011.

TECHNICAL FIELD

The present invention relates to passive thrust enhancement systemsusing circulation control applied to rotating lift/thrust devices and,in particular, to propellers.

BACKGROUND

The principle motivation in the use of circulation control has been toincrease the lifting force when large lifting forces and/or slow speedsare beneficial, such as at take-off and landing. On current fixed wingaircraft, wing flaps and leading edge slats are used during landing andon take-off. The benefit of the circulation control wing is that noadditional skin friction drag is created by the movement of conventionalsurfaces into the airflow around the wing and the lift coefficient isgreatly increased. However, as with any lifting surface, the use ofcirculation control increases the induced drag of the airfoil inproportion to the square of the increased lift coefficient.

Circulation control has also been adapted for use on rotary wingaircraft. In the early years of circulation control there were two maincritical design issues with the addition of circulation control to arotating body such as a helicopter rotor. The first because rotors onhelicopters see moderate ranges of angles of attack, 0-50 degrees causedby the inflow of air through the rotor plane. Through the study of highangles of attack in wind tunnel testing, it is possible to predict thebehavior of the rotor blade at these higher angles of attack (around20-35 degrees). The second obstacle in the prior applications ofcirculation control to a helicopter main rotor was the inability toachieve the response times necessary to effectively use circulationcontrol on a rotary wing aircraft to accommodate the asymmetry of liftexperienced during maneuvering flight. With the recent development, inthe last ten years, of smart materials, a specifically designed nearsurface piezoelectric valve (U.S. Pat. No. 6,425,553 B1) has beendeveloped, enabling the concept of using circulation control on theblades of a rotary wing aircraft. Through the use of these and similartypes of smart materials, the response time of actuation of circulationcontrol can be reduced thus lending the science to a wider array ofapplications including rotorcraft and propellers.

With the recent development of unmanned aerial vehicles (UAVs),propeller performance enhancement has become an issue. The use of activecirculation control on the propeller, though potentially beneficial iscurrently envisioned as creating technical difficulties through thesupply of air to the circulation control blowing slot. Thus, a passivethrust enhancement system in which air can be supplied to astrategically placed circulation control blowing slot can enhance theperformance of a propeller. This passive thrust enhancement system ofcirculation control for a propeller is described as the preferredembodiment of the device. However, this device functions with any fluidor gas surrounding, and passing through a similar device such as a boatpropeller or horizontal axis wind turbine.

SUMMARY

The use of the term passive system indicates that mechanical and/orelectrical power is/are not used to supply the airflow to thecirculation control sub-system. The use of this system can be applied toany rotating object which generates a fluid dynamic force in any fluidmedium, such as an aircraft propeller, a boat propeller, or a horizontalaxis wind turbine.

According to a first aspect, the passive thrust enhancement systemincludes a propeller, which includes a propeller hub, a first propellerblade, a second propeller blade, wherein the first and second propellerblades include a fluid flow channel within the first and secondpropeller blades, which is fluidly connected to a flow exit slot, and aflow capture device, which includes a plenum chamber, mounted on thepropeller hub, wherein the plenum chamber is fluidly connected to thefluid flow channels.

According to a second aspect, the passive thrust enhancement systemincludes a propeller, which includes a propeller hub, a first propellerblade, a second propeller blade, wherein the first and second propellerblades include a fluid flow channel within the first and secondpropeller blades, which is fluidly connected to a flow exit slot, a flowcapture device, which includes a plenum chamber, mounted on thepropeller hub, wherein the plenum chamber is fluidly connected to thefluid flow channels, and a flow control valve.

BRIEF SUMMARY OF THE FIGURES

FIG. 1 depicts a side view of an embodiment of a propeller with apassive thrust enhancement system.

FIG. 2 depicts a longitudinal cross section of an embodiment of apropeller blade.

FIG. 3 depicts a lateral cross section of an embodiment of a propellerblade.

FIG. 4 depicts a cross section of an embodiment of a flow capturedevice.

FIG. 5 depicts a graph of an embodiment showing the relationship betweeninlet pressure and forward velocity.

FIG. 6 depicts a graph of an embodiment showing the relationship betweenplenum pressure and blade diameter.

DETAILED DESCRIPTION

Referring to FIG. 1, an embodiment of a passive thrust enhancementsystem 100 is shown. The passive thrust enhancement system 100 includesa propeller hub 102, a first propeller blade 104, a second propellerblade 106, and a flow capture device 108. The combination of thepropeller hub 102, the first propeller blade 104, and the secondpropeller blade 106 may also be referred to as a propeller 110. Allcomponents of the passive thrust enhancement system 100 may be made outof steel, or any other material known in the art used in the making ofpropellers or other rotating lift/thrust devices.

Referring to FIGS. 2-4, in FIGS. 2 and 3, cross sections of anembodiment of the first propeller blade 104 are shown. Contained withinthe first propeller blade 104 is a fluid flow channel 200 and a flowexit slot 202. The design of the second propeller blade 106 is identicalto the first propeller blade 104. FIG. 4 shows a cross section of anembodiment of a flow capture device 108, which is mounted on thepropeller hub 102. A plenum chamber 400 is located within the flowcapture device 108. The plenum chamber 400 is fluidly connected to thefluid flow channels 200 that are located within the propeller blades.For simplicity, FIG. 4 shows one embodiment of the shape of the flowcapture device 108. The flow capture device 108 may also be a conicalstructure with straight sides, or could have sides with a continuouslyvariable curvature, or any other shape that is known in the art.

In an embodiment, the flow capture device 108 has a diameter no largerthan the diameter of the propeller hub 102, and the distance the flowcapture device 108 extends from the propeller hub 102 is between ¼ and 1diameters of the propeller hub 102. In another embodiment, the flowcapture device 108 has a diameter greater than the diameter of thepropeller hub 102 with an upper limit of 1.5 times the diameter of thepropeller hub 102. Depending on the design speed of a craft utilizingthe passive thrust enhancement system 100, other sizes and shapes offlow capture devices 108 may be used along with other distances from thepropeller hub 102, which are known in the art to not negatively impactdrag.

In an embodiment, the free stream fluid or gas “pulled” into thepropeller 110 imparts a pressure on the propeller hub 102. The pressureon the front side of the propeller 110 is greater than the localatmospheric pressure. In fluid dynamics, pressure flows from highpressure to low pressure, thus for air, or similar fluid, a localvelocity will be generated. The flow capture device 108 is attached tothe propeller hub 102, and is fluidly connected to the fluid flowchannels 200. The higher pressure at the center of the propeller hub 102is utilized to drive a flow into the flow capture device 108 through theplenum chamber 400 and through the fluid flow channels 200 to the flowexit slots 202. In addition, the fluid will be accelerated due to thecentripetal forces applied by the rotational nature of the propeller110, overcoming the friction in the fluid flow channels 200 and addingto the pressure at the flow exit slots 202.

The higher pressure at the propeller hub 102 is loosely dependent on thepropeller performance, only from the standpoint that a higher staticthrust propeller will impart more velocity to the surrounding fluid.Thus, any airfoil shape and any propeller diameter can be utilized withthe passive thrust enhancement system 100, however, the mass flow ratesupplied to the passive thrust enhancement system 100 will vary. Thevariation in mass flow may require slightly different sized flow exitslots 202 and different configurations of the fluid flow channels 200,however, the passive circulation control air, or fluid, supply conceptis unchanged.

In an embodiment, though the circulation control sections of thepropeller blades 104 and 106 have a rounded trailing edge. Additionally,the flow exit slots 202 can be placed anywhere along the span of thepropeller blades 104 and 106. In another embodiment, the flow exit slots202 are located between the propeller hub 102 and 33% of the radius ofpropeller blades 104 and 106. In another embodiment, the flow exit slots202 are located between 33% and 66% of the radius of propeller blades104 and 106 measured out from the propeller hub 102. In anotherembodiment, the flow exit slots 202 are located between 66% and 100% ofthe radius of propeller blades 104 and 106 measured out from thepropeller hub 102.

The sections of the propeller blades 104 and 106 not containing flowexit slots 202 will contain a conventional pointed trailing edgeairfoil. In an embodiment, the flow exit slots 202 can be placed at thetips of the propeller blades 104 and 106, which will vent thepressurized air out of the tips of the propeller blades 104 and 106,Venting the pressurized air in this manner may not add to the lift, butwill decrease the stagnation pressure from in front of the propeller110.

The passive nature of the passive thrust enhancement system 100 also hasan advantage of being constantly enabled, and with a simple flow controlvalve (not shown) can be restricted to times where high lift is needed(i.e. takeoff and landing or for extended flight conditions). It is alsoconceived that the flow control valve incorporated into the passivethrust enhancement system 100 can operate as a throttle to adjust themass flow supplied to the flow exit slots 202.

The inward velocity through the rotor plane can be converted to apressure force on the propeller hub 102, and through an analysis of theBernoulli Equation the relationship between pressure and velocity can befound, see Equation 1 below. With an inlet on the flow capture device108 feeding to a fluid flow channel 200 in the propeller blade,pressurized air induced from forward thrust (T) can move through theinlet at the higher pressure stagnation location, on the front of thepropeller 110, to a low pressure region (atmospheric, or less) at theflow exit slot 202. The Bernoulli Equation can be used to estimate theinlet pressure based on the assumption that the initial conditions areseen as atmospheric conditions and the forward speed of the craft iszero. FIG. 5 shows a graph of the impact of forward velocity on theinlet pressure.

P+½ρV ² +yz=constant t  Equation 1

In an embodiment, also taken into account is the internal plenumpressurization due to radial acceleration of the propeller 110 whichincreases the jet exit mass flow rate and in turn the expansion rate ofthe exiting fluid flow. The Bernoulli Equation is used to determine theexit velocity of the passive circulation control augmented propeller110. Equation 2, which is a derived expression from the Bernoulli, IdealGas, and Isentropic Equations, is used to determine the magnitude of thejet exit velocity. Equation 2 is based on the specific heat ratio (y),and the gas constant (R) of the fluid as well as the temperature (T) andinternal pressure (P) of the plenum chamber 400. FIG. 6 shows the trendsof plenum pressurization capabilities due to centripetal acceleration aswell as forward velocity, and was performed on varying size propellerblades ranging from 0.1 meters to 1.0 meter. The range of propellerblades shown in FIG. 6 are examples, and not meant as a limitation. Thepropeller blades could be any size. The trend shows a logarithmicincrease in the amount of pressure in the plenum as the rotationincreases and the propeller diameter increases. It should be noted thatthe indicated increase does not hold when the tip speed is transonic orsupersonic, that is the tip speed surpasses the local speed of sound.Also note that for simplicity, the fluid flow channel 200 is assumed tobe 0.05 by 0.05 meters, but could be any size and shape which fitswithin the propeller blade.

$\begin{matrix}{V = \sqrt{\frac{2\gamma \; {RT}}{1 - \gamma}\left\lbrack {1 - \left( \frac{P_{\infty}}{P} \right)^{\frac{\gamma - 1}{\gamma}}} \right\rbrack}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

While various embodiments of the present disclosure have been describedabove, it should be understood that they have been presented by the wayof example only, and not limitation. It will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the disclosure asdefined. Thus, the breadth and scope of the present disclosure shouldnot be limited by any of the above-described exemplary embodiments.

What is claimed is:
 1. A passive thrust enhancement system comprising: apropeller, which includes: a propeller hub; a first propeller blade; asecond propeller blade; wherein each of said first and said secondpropeller blades include a fluid flow channel within said first and saidsecond propeller blades, which is fluidly connected to a flow exit slot;and a flow capture device, which includes a plenum chamber, mounted onsaid propeller hub, wherein said plenum chamber is fluidly connected tosaid fluid flow channels.
 2. The passive thrust enhancement system ofclaim 1, wherein said flow capture device is a conical shape withstraight sides.
 3. The passive thrust enhancement system of claim 1,wherein said flow capture device is a conical shape with continuouslycurved sides.
 4. The passive thrust enhancement system of claim 1,wherein the diameter of said flow capture device is less than or equalto 150% of the diameter of said propeller hub.
 5. The passive thrustenhancement system of claim 1, wherein the distance that said flowcapture device extends from said propeller is between ¼ and 1 diametersof said propeller hub.
 6. The passive thrust enhancement system of claim1, further comprising a flow control valve.
 7. The passive thrustenhancement system of claim 1, wherein said propeller is an aircraftpropeller.
 8. The passive thrust enhancement system of claim 1, whereinsaid propeller is a boat propeller.
 9. The passive thrust enhancementsystem of claim 1, wherein said propeller is a horizontal axis windturbine.
 10. A passive thrust enhancement system comprising: apropeller, which includes: a propeller hub; a first propeller blade; asecond propeller blade; wherein each of said first and said secondpropeller blades include a fluid flow channel within said first and saidsecond propeller blades, which is fluidly connected to a flow exit slot;a flow capture device, which includes a plenum chamber, mounted on saidpropeller hub, wherein said plenum chamber is fluidly connected to saidfluid flow channels; and a flow control valve.
 11. The passive thrustenhancement system of claim 10, wherein said flow capture device is aconical shape with straight sides.
 12. The passive thrust enhancementsystem of claim 10, wherein said flow capture device is a conical shapewith continuously curved sides.
 13. The passive thrust enhancementsystem of claim 10, wherein the diameter of said flow capture device isless than or equal to 150% of the diameter of said propeller hub. 14.The passive thrust enhancement system of claim 10, wherein the distancethat said flow capture device extends from said propeller is between ¼and 1 diameters of said propeller hub.
 15. The passive thrustenhancement system of claim 10, wherein said propeller is an aircraftpropeller.
 16. The passive thrust enhancement system of claim 10,wherein said propeller is a boat propeller.
 17. The passive thrustenhancement system of claim 10, wherein said propeller is a horizontalaxis wind turbine.